1. Field of the Invention
This invention relates generally to application of light emitting elements such as light emitting diodes and laser diodes to evaluation of optical devices, amplification of optical signals, and optical communication, and more specifically to a light source apparatus suitable for production of pump light for an optical amplifier or for evaluation of a passive optical device as well as an optical amplifier and an optical communication system which includes a light source apparatus of the type just mentioned.
2. Description of the Related Art
As a result of establishment of a technology for production of silica fibers of a low loss (for example, 0.2 dB/km), an optical communication system which includes an optical fiber as a transmission line has been put into practical use. The optical communication system includes a first terminal station having an optical transmitter which outputs an optical signal, an optical fiber transmission line for transmitting the optical signal, and a second terminal station having an optical receiver which receives the optical signal transmitted thereto by the optical fiber transmission line.
In order to compensate for a loss in an optical fiber transmission line, one or more optical repeaters each having an optical amplifier for amplifying an optical signal are interposed intermediately in the optical fiber transmission line. The optical amplifier includes an optical amplification medium to which an optical signal is supplied, and means for pumping (exciting) the optical amplification medium so that the optical amplification medium may have an amplification band which includes a wavelength of the optical signal. For example, an erbium doped fiber amplifier (EDFA) for amplifying an optical signal of the wavelength band of 1.55 μm includes a doped fiber doped with erbium (Er) and having a first end and a second end, and a pump light source for supplying pump light to the doped fiber through at least one of the first and second ends. When an optical signal to be amplified is supplied to the doped fiber through the first end, then the optical signal is amplified in the doped fiber in accordance with the principle of stimulated emission, and the amplified optical signal is outputted through the second end.
Since an EDFA requires a pump light source for pumping a doped fiber in this manner, an electronic circuit for supplying power to the pump light source is provided additionally for the EDFA. On the other hand, where an optical fiber transmission line is laid on the bottom of the sea, since an optical repeater having the EDFA must be kept sunk on the bottom of the sea, maintenance of the optical repeater is very difficult. According, it is required for an optical fiber communication system which includes an EDFA that the maintenance be easy.
As a pump light source for an optical amplifier, a light emitting element such as a light emitting diode or a laser diode is used. In order to raise the optical power outputted from an optical amplifier, the maximum optical power of pump light to be supplied to a doped fiber should be raised. Since the output optical power of one light emitting element is limited, in order to obtain a higher optical power, two light emitting elements which output first pump light and second pump light having different wavelengths from each other are used and the first pump light and the second pump light are wavelength division multiplexed.
For example, in order to effect wavelength division multiplexing in a pump band of the 1.48 μm wavelength band (1.45 μm to 1.50 μm) which is adopted by the EDFA, a spectrum width of approximately 20 nm is required for each of the first pump light and the second pump light, and the dispersion of the center wavelength must be restricted to approximately ±5 nm. However, the spectrum of a light emitting element is liable to be dispersed due to an error in production or the like, and there is a problem that the yield of light emitting elements which can be used for wavelength division multiplexing is low.
Incidentally, in order to evaluate a passive optical device represented by an optical multiplexer which is used for wavelength division multiplexing, a light source apparatus for obtaining a beam of light having a desired center wavelength is required. While a variable wavelength light source is known as one of light source apparatus of the type mentioned, the variable wavelength light source has a problem in that it is expensive. Further, while it may be proposed to apply a band-pass filter to a beam of light outputted from a light emitting diode having a comparatively broad spectrum to obtain a beam of light having a center wavelength necessary for measurement, since the beam of light obtained in this instance is reduced in power, the dynamic range in measurement is decreased.
It is to be noted that, as a technique which seems to pertain to the light source apparatus of the present application, an apparatus disclosed in Japanese Patent Laid-Open Application No. Heisei 7-45890 or Japanese Patent Laid-Open Application No. Showa 62-154685 is known. The former discloses an external resonator type semiconductor laser wherein an external resonator for a semiconductor laser is formed using a diffraction grating and driving current for the semiconductor laser is controlled in accordance with diffracted light from the diffraction grating. The latter discloses a light source for wavelength multiplex communication wherein a single common diffraction grating is applied to a plurality of semiconductor lasers.